Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle, which is configured for implementing at least ten forward speeds using a minimum number of configurations, improving power transmission performance and fuel efficiency by increasing a span of a gear shift ratio, and ensuring uniformity (linearity of graph) of ratios between gear shift stages.
Description of Related art
Research on realizing more shift-stages of an automatic transmission are undertaken to achieve enhancement of fuel consumption and better drivability, and recently, increase of oil price is triggering a hard competition in enhancing fuel consumption of a vehicle.
In this sense, research on an engine has been undertaken to achieve weight reduction and to enhance fuel consumption by so-called downsizing and research on an automatic transmission has been performed to simultaneously provide better drivability and fuel consumption by achieving more shift stages.
In order to achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.
Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for better efficiency to be derived by a smaller number of parts.
In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages is under investigation.
An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to seven control elements (frictional elements), and may easily become lengthy, thereby deteriorating installability.
In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements is sometimes attempted. However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.
In addition, considering that gear ratio spans of recently developed eight-speed automatic transmissions are typically between 6.5 and 7.5, fuel consumption enhancement is not very large, and in the case of a gear ratio span of an eight-speed automatic transmission having a level above 9.0, it is difficult to maintain step ratios between adjacent shift stages to be linear, by which driving efficiency of an engine and drivability of a vehicle deteriorated.
Thus, researches studies are under investigation for developing a high efficiency automatic transmission having nine or more speeds.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.